A comparison was made of electrocardiographic findings in 107 first-degree relatives of patients with hypertrophic cardiomyopathy without any clinical and echocardiographic signs of the disease and 188 healthy persons with a negative family history. A significantly larger number of electrocardiographic signs of left ventricular hypertrophy (P less than 0.05) and abnormal Q wave (P less than 0.005) was shown in the group of the relatives. Abnormalities of the R wave in V1-3 and of the ST-T segment were also more frequent in this group, but the difference is not statistically significant. In all, electrocardiographic abnormalities were found in 13 of 107 asymptomatic relatives of the patients with hypertrophic cardiomyopathy. These relatives had normal clinical and echocardiographic findings. These 13 patients underwent long-term follow-up (4.5-8 years). Typical hypertrophic cardiomyopathy developed in two patients (an increase in the myocardial thickness from 6 to 15 m in six years and from 8 to 13 mm in 4.5 years, respectively) which was accompanied by progression of the electrocardiographic findings. Electrocardiography is the only commonly available method which may reveal the latent forms of hypertrophic cardiomyopathy at the stage when neither myocardial hypertrophy nor other signs of the disease are expressed. Longitudinal follow-up is necessary for all the relatives of the patients with hypertrophic cardiomyopathy who have abnormal or borderline electrocardiographic findings. A normal echocardiogram cannot exclude the disease at this stage.